1. Field of the Invention
The present invention relates to a die for bending a composite flange having a stretch portion and a straight portion.
2. Description of the Prior Art
An example of a member having a composite flange is a suspension member of an automobile. As illustrated in FIGS. 9 and 10, a suspension member 2 extends longitudinally and is formed by bending a flat plate into a channel member with a substantially inverted U-shaped transverse cross section. Each side of the inverted U constitutes a composite flange 4 which has a stretch portion 6 and a straight portion 8. In this instance, a stretch portion is defined as a flange portion which is bent along a curved bending line 10 and therefore is stretched when bent to generate a tensile stress along the bending line 10 due to a difference between an arc length d2 at the bending line 10 and an arc length d2 along a radially inside edge when the member is in a flat state. A straight portion is defined as a flange portion which is bent along a straight bending line 12 and therefore is not stretched in a longitudinal direction. Due to the tensile stress generated in the stretch portion 6, a warp 14 will be generated in a free edge of the stretch portion 6, as illustrated in FIG. 10.
FIGS. 8a, b, and c illustrate a conventional method for forming having an inverted U-shaped cross section and a stretch flange portion. In the method, a flat plate is first drawn to form a member with an inverted U-shaped cross section in FIG. 8a, then by cam-restriking is compressed into a member 18 with a more exact configuration (FIG. 8b), and is finally cam-trimmed to cut off free edge portions 20 of the composite flanges which may be warped, as shown in FIG. 8c.
With the prior art method, the various types of cams that are necessary result in increased size and cost of the press machine.
For the purpose of reducing the number of forming cams and dies so as to make the forming machine compact, an alternative forming method as shown in FIGS. 5a, b, and c has been developed by the inventor. In this method, a flat plate is drawn into a semi-channel member with an intermediate hat-like cross section 22 (FIG. 5a), then is press-trimmed at brim portions 24 thereof (FIG. 5b), and then is finally bent by bending dies into a full channel member with an inverted U-shaped cross section 26 (FIG. 5c). However, in this method, it is necessary to develop a bending die capable of preventing a composite flange from warping during forming from the intermediate hat-like cross section to the final inverted U-shaped cross section. More particularly, if the composite flange were bent using a conventional die 28 as shown in FIG. 7, the stretch portion 30 and the straight portion 32 of the composite flange, as shown in FIG. 6, would begin to be bent at the same time. In this instance, the straight portion 32 would be bent more than the stretch portion 30, because bending of the stretch portion 30 would be suppressed due to the longitudinal tensile stress generated in the stretch portion 30. As a result, a warp 34 would occur at the free edge of the stretch portion 30.